Image device and methods of forming the same

ABSTRACT

A method of forming of an image sensor device includes an isolation well formed in a pixel region of a substrate. The isolation well has a first conductivity type. A gate stack is formed over the isolation well on the substrate. A mask layer is formed over the isolation well and covering at least a majority portion of the gate stack. A plurality of dopants is implanted in the pixel region, using the gate stack and the mask layer as masks, to form doped isolation features. The plurality of dopants has the first conductivity type. A source region and a drain region are formed on opposite sides of the gate stack in the substrate. The source region and the drain region have a second conductivity type opposite to the A conductivity.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority of U.S. Provisional PatentApplication No. 61/642,883, filed on May 4, 2012, which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to an image sensor device and methods forforming an image sensor device.

BACKGROUND

An image sensor device is one of the building blocks in a digitalimaging system such as a digital still or video camera. An image sensordevice includes a pixel array (or grid) for detecting light andrecording intensity (brightness) of the detected light. The pixel arrayresponds to the light by accumulating a charge—for example, the morelight, the higher the charge. The accumulated charge is then used (forexample, by other circuitry) to provide a color and brightness for usein a suitable application, such as a digital camera. One type of imagesensor device is a backside illuminated (BSI) image sensor device. BSIimage sensor devices are used for sensing a volume of light projectedtowards a backside surface of a substrate (which supports the imagesensor circuitry of the BSI image sensor device). The pixel grid islocated at a front side of the substrate, and the substrate is thinenough so that light projected towards the backside of the substrate canreach the pixel grid. BSI image sensor devices provide a reduceddestructive interference, as compared to front-side illuminated (FSI)image sensor devices.

Integrated circuit (IC) technologies are constantly being improved. Suchimprovements frequently involve scaling down device geometries toachieve lower fabrication costs, higher device integration density,higher speeds, and better performance. Along with the advantagesrealized from reducing geometry size, improvements are being madedirectly to the image sensor devices.

Due to device scaling, improvements to image sensor device technologyare continually being made to further improve image quality of imagesensor devices. Although existing image sensor devices and methods offabricating image sensor devices have been generally adequate for theirintended purposes, as device scaling down continues, they have not beenentirely satisfactory in all respects.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure may be understood from the followingdetailed description and the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a top view of an image sensor device according to variousembodiments of this disclosure.

FIG. 2A is an enlarged top view of a pixel region in the image sensordevice of FIG. 1.

FIG. 2B is an enlarged top view of a portion of the pixel region in theimage sensor device of FIG. 2A.

FIG. 2C is a cross sectional view of the pixel region along line B-B′ inFIG. 2A and a periphery region of the image sensor device according toone or more embodiments of this disclosure.

FIG. 3 is a flowchart of a method of forming an image sensor deviceaccording to one or more embodiments of this disclosure.

FIGS. 4A to 7B are top views and cross-sectional views of a portion ofthe pixel region of the image sensor device in FIG. 2A at various stagesof manufacture according to various embodiments of the method of FIG. 3.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific examples of components are arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, examples and are not intended to be limiting. Moreover, theformation of a first feature over a second feature in the descriptionthat follows may include embodiments in which the first and secondfeatures are formed in direct contact, and may also include embodimentin which additional features may be formed interposing the first andsecond features, such that the first and second features may not be indirect contact. Further still, references to relative terms such as“top”, “front”, “bottom”, and “back” are used to provide a relativerelationship between elements and are not intended to imply any absolutedirection. Various features may be arbitrarily drawn in different scalesfor simplicity and clarity.

FIG. 1 is a top view of an image sensor device 100 according to variousaspects of the present disclosure. In the depicted embodiment, the imagesensor device is a backside illuminated (BSI) image sensor device. Theimage sensor device 100 includes an array of pixel regions 101. Eachpixel region 101 is arranged into a column (for example, C₁ to C_(x))and a row (for example, R₁ to R_(y)). The term “pixel region” refers toa unit cell containing features (for example, a photodetector andvarious circuitry), which may include various semiconductor devices forconverting electromagnetic radiation to an electrical signal.Photodetectors in the pixel regions 101 may include photodiodes,complimentary metal-oxide-semiconductor (CMOS) image sensors, chargedcoupling device (CCD) sensors, active sensors, passive sensors, and/orother sensors. The pixel regions 101 may be designed having varioussensor types. For example, one group of pixel regions 101 may be CMOSimage sensors and another group of pixel regions 101 may be passivesensors. In the depicted embodiment, each pixel region 101 may include aphotodetector, such as a photogate-type photodetector, for recording anintensity or brightness of light (radiation). Each pixel region 101 mayalso include various semiconductor devices, such as various transistorsincluding a transfer transistor, a reset transistor, a source-followertransistor, a select transistor, other suitable transistor, orcombinations thereof. Additional circuitry, inputs, and/or outputs maybe in a periphery region of the image sensor device 100. Thosecircuitry, inputs, and/or outputs in the periphery region are coupled tothe pixel regions 101 to provide an operation environment for the pixelregions 101 and support external communications with the pixel regions101. For simplicity, an image sensor device including a single pixelregion is described in the present disclosure; however, typically anarray of such pixel regions may form the image sensor device 100illustrated in FIG. 1.

FIG. 2A is an enlarged top view of a pixel region 101 in the imagesensor device 100 on a substrate (not illustrated in FIG. 2A). The pixelregion 101 refers to a unit cell containing at least one photodetector106 and various circuitry for converting electromagnetic radiation to anelectrical signal. In the depicted embodiment, the photodetector 106includes a photodiode for recording an intensity or brightness of light(radiation). The pixel region 101 may contain various transistorsincluding a transfer transistor 110, a reset transistor 112, asource-follower transistor 114, a select transistor 116, or othersuitable transistors, or combination thereof. The pixel region 101 mayalso include various doped regions in the substrate, for example dopedregion 118A, 118B and 120. The doped regions 118A and 118B areconfigured as source/drain regions of previous mentioned transistors.The doped region 120 is also referred as a floating diffusion region120. The floating diffusion region 120 is between a gate stack of thetransfer transistor 110 and a gate stack of the reset transistor 112,and is one of source/drain regions for transfer transistor 110 and thereset transistor 112. A conductive feature 132 overlaps a portion of agate stack of the source-follower transistor 114 and connects to thefloating diffusion region 120. The image sensor device 100 also includesvarious isolation features (for example, doped isolation feature 108 inFIG. 2A and dielectric isolation feature 126 in FIG. 2C) formed in thesubstrate to isolate various regions of the substrate to prevent leakagecurrents between various regions. In the depicted embodiment, a dopedisolation feature 108 is formed in the pixel region 101 to isolate thephotodetector 106, the transfer transistor 110, the reset transistor112, the source-follower transistor 114 and the select transistor 116.FIG. 2B illustrates an enlarged top view of a portion 200 of the pixelregion 101. A corresponding gate stack of the source-follower transistor114 is disposed on the pixel region 101. The doped regions 118Bconfigured as source/drain regions are disposed adjacent the gate stackof the source-follower transistor 114. The doped isolation feature 108surrounds the doped regions 118B and the gate stack of thesource-follower transistor 114.

FIG. 2C is a cross sectional view of the pixel region 101 along lineB-B′ in FIG. 2A and a periphery region 102 of the image sensor device100. The image sensor device 100 includes a substrate 104 having a frontsurface 104A and a back surface 104B. In the depicted embodiment, thesubstrate 104 is a semiconductor substrate including silicon.Alternatively or additionally, the substrate 104 includes anotherelementary semiconductor, such as germanium and/or diamond; a compoundsemiconductor including silicon carbide, gallium arsenic, galliumphosphide, indium phosphide, indium arsenide, and/or indium antimonide;an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs,GaInP, and/or GaInAsP; or combinations thereof. The substrate 104 may bea semiconductor on insulator (SOI). The substrate 104 may have variousdoping configurations depending on design requirements (e.g., p-typesubstrate or n-type substrate). In some embodiments, the p-type refersto making holes as majority charge carriers in a semiconductor material,and the n-type refers to making electrons as majority charge carriers ina semiconductor material. In the depicted embodiment, the substrate 104is a p-type substrate. P-type dopants that the substrate 104 is dopedwith include boron, gallium, indium, other suitable p-type dopants, orcombinations thereof.

The pixel region 101 includes at least one photodetector 106, such as aphotodiode, that includes a light-sensing region 106A and a pinned layer106B. The light-sensing region 106A is a doped region having a firstconductivity type of dopants formed in the substrate 104, specificallyalong the front surface 104A of the substrate 104. In the depictedembodiment, the light-sensing region 106A is an n-type doped region. Thepinned layer 106B is a doped layer overlapping the light-sensing region106A at the front surface 104A of the substrate 104. The pinned layer106 has a conductivity type of dopants opposite to the light-sensingregion 106A. In the depicted embodiment, the pinned layer 106B is ap-type implanted layer.

The pixel region 101 further includes various transistors, such as thetransfer transistor 110 (shown in FIG. 2A), the reset transistor 112(shown in FIG. 2A), the source-follower transistor 114 and the selecttransistor 116 (shown in FIG. 2A). Each transistor has a correspondinggate stack disposed over the front surface 104A of the substrate 104. Inthe depicted embodiment, the gate stack of the source-followertransistor 114 overlies an isolation well region 109. A top surface ofthe isolation well region 109 is away from the front surface 104A with adistance W₁. The distance W₁ is in a range from about 1000 Å to about3000 Å. A bottom surface of the isolation well region 109 extendsfurther into the substrate 104 toward the back surface 104B. Theisolation well region 109 has a second conductivity type opposite to thefirst conductivity type of the light-sensing region 106A. In thedepicted embodiment, the isolation well region 109 is a p-type dopedregion. A dosage used for the isolation well region 109 is from about1×10¹¹ to 3×10¹¹ atoms/cm³. The isolation well region 109 surrounds thelight-sensing region 106A of photodetector 106. The gate stack of eachtransistor includes a gate dielectric layer and a gate electrode layer.The gate dielectric layer includes a dielectric material, such assilicon oxide, a high dielectric constant (high-k) dielectric material,other dielectric material, or combinations thereof. Examples of high-kdielectric material include HfO₂, HfSiO, HfSiON, HfTaO, HfTiO, HfZrO,zirconium oxide, aluminum oxide, hafnium dioxide-alumina (HfO₂—Al₂O₃)alloy or combinations thereof. The gate electrode layer includespolysilicon and/or a metal including Al, Cu, Ti, Ta, W, Mo, TaN, NiSi,CoSi, TiN, WN, TiAl, TiAlN, TaCN, TaC, TaSiN or combinations thereof.

The periphery region 102 may include readout circuitry and/or controlcircuitry coupled to the pixel region 101 to provide an operationenvironment for the pixel region 101. In the depicted embodiment, a PMOStransistor 122 and a NMOS transistor 124 are shown. The PMOS transistor122 includes a gate stack 122A and source/drain regions 122B havingp-type conductivity formed in an n-type well 122C. The NMOS transistor124 includes a gate stack 124A and source/drain regions 124B havingn-type conductivity formed in a p-type well 124C.

The image sensor device 100 further includes a plurality of dopedisolation features 108 formed in substrate 104 of the pixel region 101and a plurality of dielectric isolation features 126 formed in substrate104 of the periphery region 102. The doped isolation features 108 andthe dielectric isolation features 126 isolate various regions of thesubstrate 104 to prevent leakage currents between various regions. Inthe depicted embodiment, the doped isolation features 108 and thedielectric isolation features 126 isolate the PMOS transistor 122 andthe NMOS transistor 124, the photodetector 106, the transfer transistor110 (shown in FIG. 2A), the reset transistor 112 (shown in FIG. 2A), thesource-follower transistor 114 and the select transistor 116 (shown inFIG. 2A).

Each of the doped isolation features 108 has a depth D₁ extending formthe front surface 104A into the substrate 104. The depth D₁ is in arange from about 1000 Å to about 3000 Å. The doped isolation feature 108has the second conductivity type as the isolation well region 109. Thedepth D₁ of doped isolation features 108 is substantially equal to thedistance W₁ of the isolation well region 109 to the front surface 104Aof the substrate 104. The doped isolation features 108 and the isolationwell regions 109 surround the light-sensing region 106A of thephotodetector 106 to prevent horizontal leakage paths between thephotodetector 106 and other regions. In the depicted embodiment, dopedisolation feature 108 is a p-type doped region. P-type dopants of thedoped isolation feature 108 include boron (B), BF₂, gallium, indium,other suitable p-type dopants or combination thereof. A dosage used forthe dopants is about from about 2×10¹² to about 8×10¹² atoms/cm³.Alternatively, when the isolation well region 109 is an n-type dopedregion, doped isolation feature 108 is also an n-type doped region.N-type dopants of the doped isolation feature 108 include phosphorus,arsenic, other suitable n-type dopants or combination thereof.

The dielectric isolation features 126 include silicon oxide, siliconnitride, silicon oxynitride, other insulating material, or combinationthereof. Each of the dielectric isolation features 126 has a depth D₂extending form the front surface 104A into the substrate 104. The depthD₂ is in a range of about 2000 Å to about 3000 Å. The formation ofdielectric isolation features 126 may include a photolithographyprocess, an etching process to etch a trench from the front surface 104Ainto the substrate 104 and a deposition process to fill the trench (forexample, by using a chemical vapor deposition process) with dielectricmaterial.

The image sensor device 100 further includes a multilayer interconnect(MLI) 128 disposed over the front surface 104A of the substrate 104,including over the photodetector 106. The MLI 128 is coupled to variouscomponents of the image sensor device 100, for example the photodetector106, such that the various components of the image sensor device 100 areoperable to properly respond to illuminated light (imaging radiation).The MLI 128 includes various conductive features 130 and 132, which maybe vertical interconnects 130, such as contacts and/or vias 130, andhorizontal interconnects 132, such as lines 132. The various conductivefeatures 130 and 132 include conductive materials, such as aluminum,aluminum/silicon/copper alloy, titanium, titanium nitride, tungsten,polysilicon, metal silicide, or combinations thereof.

The various conductive features 130 and 132 of the MLI 128 areinterposed in an interlayer dielectric (ILD) layer 134. The ILD layer134 may include silicon dioxide, silicon nitride, silicon oxynitride,TEOS oxide, phosphosilicate glass (PSG), borophosphosilicate glass(BPSG), fluorinated silica glass (FSG), carbon doped silicon oxide,BLACK DIAMOND® (Applied Materials of Santa Clara, Calif.), amorphousfluorinated carbon, high dielectric constant (low-k) dielectricmaterial, polyimide, or combinations thereof. The ILD layer 134 may havea multilayer structure.

A carrier wafer 136 is disposed over the front surface 104A of thesubstrate 104. In the depicted embodiment, the carrier wafer 136 isbonded to the MLI 128. The carrier wafer 136 includes silicon or glass.The carrier wafer 136 can provide protection for the various features(such as the photodetector 106) formed on the front surface 104A of thesubstrate 104, and can also provide mechanical strength and support forprocessing the back surface 104B of the substrate 104.

The image sensor device 100 further includes a doped layer 138 disposedat the back surface 104B of the substrate 104. The doped layer 138 isformed by an implantation process, diffusion process, annealing processor combinations thereof. In the depicted embodiment, the doped layer 138includes p-type (second conductivity type) dopants, such as boron,gallium, indium or combinations thereof. The doped layer 138 has adopant depth, d, that extends into the substrate 104 from the backsurface 104B of the substrate 104. The dopant depth, dopantconcentration, dopant profile, or combination thereof of the doped layer138 may be selected to optimize image quality by increasing quantumefficiency, reducing dark current or reducing white pixel defects.

The image sensor device 100 may further include an antireflective layer140, a color filter 142 and a lens 144 disposed over the back surface104B of the substrate 104. The antireflective layer 140 includes adielectric material, such as silicon nitride or silicon oxynitride.

The color filter 142 is disposed over the antireflective layer 140, andis aligned with the light-sensing region 106A of the photodetector 106.The color filter 142 is designed to filter out visible light out of apredetermined wavelength. For example, the color filter 142 may filterout visible light except the light of a red wavelength, a greenwavelength, or a blue wavelength to the photodetector 106. In anexample, the color filter 142 includes a dye-based (or pigment-based)polymer for filtering out a specific frequency band (for example, adesired wavelength of light).

The lens 144 is disposed over the color filter 142 and is also alignedwith the light-sensing region 106A of the photodetector 106. The lens144 may be in various positional arrangements with the photodetector 106and color filter 142, such that the lens 144 focuses an incidentradiation 146 on the light-sensing region 106A of the photodetector 106.Alternatively, the position of the color filter layer 142 and the lens144 may be reversed, such that the lens 144 is disposed between theantireflective layer 140 and color filter 142.

In an operation of the image sensor device 100 according to one or moreembodiments, the image sensor device 100 is designed to receive aradiation 146 traveling towards the back surface 104B of the substrate104. The lens 144 directs the incident radiation 146 to the color filter142. The incident radiation 146 then passes from the color filter 142through the antireflective layer 140 to the substrate 104 andcorresponding photodetector 106, specifically to light-sensing region106A. When exposed to the incident radiation 146, the photodetector 106responds to the incident radiation 146 by accumulating charges.Referring back to FIG. 2A, the charges are transferred from thephotodetector 106 to the floating diffusion region 120 when the gate oftransfer transistor 110 is turned on. Through the connection of theconductive feature 132 (shown in FIG. 2A), the source-followertransistor 114 may convert the charges from the floating diffusionregion 120 to voltage signals. The select transistor 116 may allow asingle row of the pixel array to be read by read-out electronics. Thereset transistor 112 acts as a switch to reset the floating diffusionregion 120. When the reset transistor 112 is turned on, the floatingdiffusion region 120 is effectively connected to a power supply clearingall accumulated charges.

FIG. 3 is a flowchart of a method 300 of forming an image sensor deviceaccording to one or more embodiments of this disclosure. The flowchartof the method 300, at operation 301, a substrate has a pixel region. Anisolation well region is formed in the pixel region. The isolation wellregion has a first conductivity type. In the depicted embodiment, thefirst conductivity type is a p-type polarity. Alternatively, the firstconductivity type is an n-type polarity. In one embodiment, theisolation well region is away form a front surface of the substrate witha distance W₁. The distance W₁ is in a range from about 1000 Å to about3000 Å. Next, the method 300 continues with operation 302 in which agate stack is formed over the isolation well region on the substrate.The method 300 continues with operation 303 in which a mask layer isformed over the isolation well region and covering at least a majorityportion of the gate stack. The method 300 continues with operation 304in which a plurality of dopants is implanted into the pixel region notcovered by the mask layer to form doped isolation features surroundingthe gate stack. The plurality of dopants has the first conductivitytype. In the depicted embodiment, the first conductivity type is ap-type polarity. Alternatively, the first conductivity type is an n-typepolarity. The method 300 continues with operation 305 in whichsource/drain regions are formed on opposite sides of the gate stack inthe substrate. The source/drain regions have a second conductivity typeopposite to the first conductivity type. In the depicted embodiment, thesecond conductivity type is an n-type polarity. Alternatively, thesecond conductivity type is a p-type polarity. Further, it is understoodthat additional steps can be provided before, during, and after themethod 300.

FIGS. 4A to 7B are top views and cross-sectional views of the portion200 of the pixel region 101 in image sensor device 100 at various stagesof manufacture according to various embodiments of the method of FIG. 3.Various figures have been simplified for a better understanding of theinventive concepts of the preset disclosure.

Referring back to FIG. 3, the method 300 proceeds from operation 301 andcontinues to operation 302.

FIG. 4A is a top view of the portion 200 of the pixel region 101 afterperforming operations 301 and 302. FIG. 4B is a cross sectional view ofthe portion 200 along line A-A′ in FIG. 4A. A substrate 104 has a frontsurface 104A and a back surface 104B. The front surface 104A is definedby a first axis (along line B-B′) and a second axis (along line A-A′)that is perpendicular to the first axis. The substrate 104 is asemiconductor substrate including silicon. In the depicted embodiment,the substrate 104 is a p-type silicon substrate. P-type dopants that thesubstrate 104 is doped with include boron, gallium, indium, othersuitable p-type dopants, or combinations thereof. Alternatively, thesubstrate 104 includes suitable materials mentioned in the previousparagraphs.

An isolation well region 109 having a first conductivity type is formedin the pixel region 101. The isolation well region 109 is beneath thefront surface 104A of the substrate with a distance W₁. The distance W₁is in a range from about 1000 Å to about 3000 Å. A bottom surface of theisolation well region 109 extends into the substrate 104 toward the backsurface 104B. The isolation well region 109 is formed by lithographypatterning and implantation process. In the depicted embodiment, theisolation well region 109 is a p-type doped region. P-type dopants ofthe isolation well region 109 include as boron, gallium, indium orcombinations thereof. A dosage used for the dopants is from about 1×10¹¹atoms/cm³ to about 3×10¹¹ atoms/cm³. Alternatively, the isolation wellregion 109 is an n-type doped region including n-type dopants such asphosphorus, arsenic, other suitable n-type dopants or combinationthereof.

Next, a gate stack is formed over the isolation well region 109 and onthe front surface 104A of the substrate 104. In the depicted embodiment,the gate stack of the source-follower transistor 114 is shown forillustration purpose. The gate stack of the source-follower transistor114 is also referred to as the gate stack 114. The gate stack 114 has alength X₁ along the first axis (along line B-B′) and a length Y₁ alongthe second axis (along line A-A′). The gate stack 114 is formed withinthe isolation well region 109. The length Y₁ of the gate stack 114 isdefined as a channel length of the source-follower transistor 114. Thegate stack 114 are formed by suitable process, including deposition,lithography patterning and etching processes. The gate stack 114includes a gate dielectric layer and a gate electrode layer. The gatedielectric layer includes a dielectric material, such as silicon oxide,a high-k dielectric material, other dielectric material, or combinationsthereof. Examples of high-k dielectric material include HfO₂, HfSiO,HfSiON, HfTaO, HfSiO, HfZrO, zirconium oxide, aluminum oxide, hafniumdioxide-alumina (HfO₂—Al₂O₃) alloy or combinations thereof. The gateelectrode layer includes polysilicon and/or a metal including Al, Cu,Ti, Ta, W, Mo, TaN, NiSi, CoSi, TiN, WN, TiAl, TiAlN, TaCN, TaC, TaSiNor combinations thereof.

Referring back to FIG. 3, the method 300 continues with operation 303. Amask layer is formed over the isolation well region and covers at leasta majority portion of the gate stack.

FIG. 5 is a top view of the portion 200 of the pixel region 101 afterperforming operation 303. A mask layer 202 (shown in dashed line) isformed over the isolation well region 109 and covering at least amajority portion of the gate stack 114. The mask layer 202 is configuredto define an active area 203 of the source-follower transistor 114.Source/drain regions (shown as 118B in FIG. 7A) of the source-followertransistor 114 could be formed within the active area 203 in thefollowing processes. The active area 203 covered by the mask layer 203has a length X₂ along the first axis (along line B-B′) and a length Y₂along the second axis (along line A-A′). The length X₂ of the activearea 203 is defined as a channel width of the source-follower transistor114. The length X₂ of the active area 203 is no longer than the lengthX₁ of the gate stack 114 along the first axis (along line B-B′). Thesubsequently formed source/drain regions (shown as 118B in FIG. 7A) areconfined with the active area 203. In one embodiment, the entire gatestack 114 is covered by the mask layer 202 and edges of the mask layer202 are substantially aligned with edges of the gate stack 114 along thefirst axis (along line B-B′). In another embodiment, a minority portionof the gate stack 114 is not covered by the mask 202 along the firstaxis (along line B-B′) as shown in FIG. 5. The mask layer 202 is formedby lithography patterning processes to define a feature over theisolation well region 109 and the gate stack 114. The lithographypatterning processes include photoresist coating, soft baking, maskaligning, exposure, post-exposure baking, developing the photoresist,rising, drying (e.g., hard baking) or combination thereof.

Referring back to FIG. 3, the method 300 continues with operation 304. Aplurality of dopants is implanted into the pixel region not covered bythe mask layer to form doped isolation features surrounding the gatestack. The plurality of dopants has the first conductivity type as theisolation well region. In the depicted embodiment, the firstconductivity type is a p-type polarity. Alternatively, the firstconductivity type is an n-type polarity.

FIG. 6A is a top view of the portion 200 of the pixel region 101 afterperforming operation 304. FIG. 6B is a cross sectional view of theportion 200 along line A-A′ in FIG. 6A. FIG. 6C is the cross sectionalview of the portion 200 along line B-B′ in FIG. 6A. Referring to FIGS.6B and 6C, a plurality of dopants 204 is implanted into the portion 200of the pixel region 101 not covered by the mask layer 202 to form dopedisolation features 108. The plurality of dopants has the firstconductivity type as the isolation well region 109. Doped isolationfeatures 108 are formed to surround the active area 203 of thesource-follower transistor 114 covered by the mask layer 202. In thedepicted embodiment, doped isolation feature 108 is a p-type dopedregion. P-type dopants of the doped isolation feature 108 include boron(B), BF₂, gallium, indium, other suitable p-type dopants or combinationthereof. A dosage used for the dopants is from about 2×10¹² to about8×10¹² atoms/cm³. The implantation is performed with a tilt angle θ,between a plane parallel to the front surface 104A and an incident beamof the implantation, from about 75 to about 90 degrees. Each of thedoped isolation features 108 has a depth D₁ extending form the frontsurface 104A into the substrate 104. The depth D₁ is in a range of about1000 Å to about 3000 Å. The depth D₁ of doped isolation features 108 issubstantially equal to the distance W₁ of the isolation well region 109to the front surface 104A of the substrate 104. The doped isolationfeatures 108 and the isolation well regions 109 surround the active area203 of the source-follower transistor 114 and also surround thelight-sensing region 106A of the photodetector 106 (shown in FIGS. 2Aand 2C). The possible horizontal leakage paths between the photodetector106 and the source-follower transistor 114 may be eliminated. When thedepth D₁ is less than 1000 Å, the doped isolation feature 108 could notelectrically isolate various regions. Hence, the device performance ofthe image sensor device 100 may reduce. When the depth D₁ is larger than3000 Å, the mask layer 202 does not effectively protect the underlying egate stack 114 from damage during the high energy implantation processto achieve the depth D₁.

Due to the implantation is performed with the tilt angle θ, the dopedisolation feature 108 may extends under the gate stack 114 along thefirst axis (along line B-B′) with a length L form an edge 114E of thegate stack 114 as shown in FIG. 6C. The gate stack 114 is over a portionof the doped isolation feature 108 with the length L. The length L isless than about 0.1 μm. A profile of the doped isolation feature 108 hasa convex portion from the edge 114E of the gate stack 114 along thefirst axis (along line B-B′). Also, a top corner108C of the profile isaligned with the edge 114E of the gate stack 114. When the tilt angle θis larger than 90 degrees, the gate stack 114 does not overlie a portionof the doped feature 108. Edge portions of the gate stack 114 may not bewell isolated, and the device performance of the image sensor device 100may not be well controlled. In some situations, there is a currentleakage path along the edge 114E (along the second axis line A-A′direction)of the gate stack 114 which may lead to a short circuitbetween the later formed source/drain regions during the operation ofthe image sensor device 100. When the tilt angle θ is less than 75degrees, the doped isolation feature 108 may extend too much under thegate stack 114 and the channel width X₂ will be shortened and the deviceperformance will be affected. In some embodiments, the doped isolationfeature 108 is an n-type doped region including n-type dopants such asphosphorus, arsenic, other suitable n-type dopants or combinationthereof.

Referring back to FIG. 3, the method 300 continues with operation 305 inwhich source/drain regions are formed on opposite sides of the gatestack in the substrate. The source/drain regions have a secondconductivity type opposite to the first conductivity type. In thedepicted embodiment, the second conductivity type is an n-type polarity.Alternatively, the second conductivity type is a p-type polarity.

FIG. 7A is a top view of the portion 200 of the pixel region 101 afterperforming operation 305. FIG. 7B is a cross sectional view of theportion 200 along line A-A′ in FIG. 7A. Source/drain regions 118B areformed on opposite sides of the gate stack 114 above the isolation wellregion 109 on the substrate 104. The source/drain regions 118B have thelength X₂ along the first axis (along line B-B′). The length X₂ of thesource/drain regions 118B is defined as the channel width of thesource-follower transistor 114. As mentioned in previous paragraph, themask layer 202 defines the active area 203 and the source/drain regions118B are confined with the active area 203. The length X₂ of the activearea 203 is not greater than the length X₁ of the gate stack 114 alongthe first axis (along line B-B′). As shown in FIG. 7A, edges 118C of thesource/drain regions 118B do not protrude over edges 114E of the gatestack 114 along the first axis (along line B-B′). With thisconfiguration, the leakage current path will not go from drain regionalong the edge 114E of the gate stack 114 to source region during theoperation of the image sensor device 100. This configuration preventsthe image sensor device 100 from electrical short. Hence, the currentbetween the source/drain regions 118B is confined within the gate width(channel width) X₂ along the second axis (along line A-A′). The deviceperformance may be accurately controlled.

The source/drain regions 118B have a second conductivity type oppositeto the first conductivity type of the doped isolation feature 108 andthe isolation well region 109. Also, the floating diffusion region 120in FIG. 2A has the second conductivity type to be configured as one ofsource/drain regions for transfer transistor 110 and the resettransistor 112. In the depicted embodiment, the transistor 114 is a NMOStransistor. Source/drain regions 118B are n-type doped regions. N-typedopants of the source/drain regions 118B include phosphorus, arsenic,other suitable n-type dopants or combination thereof. Alternatively, thetransistor 114 is a PMOS transistor. Source/drain regions 118B arep-type doped regions. P-type dopants of the source/drain regions 118Binclude boron (B), BF₂, gallium, other suitable p-type dopants orcombination thereof.

It is understood that additional steps can be provided before, during,and after the operation 305 of the method 300. For example, as shown inFIG. 2C. The image sensor device 100 further includes a photodetector106, such as a photodiode, that includes a light-sensing region 106A anda pinned layer 106B. The light-sensing region 106A is a doped regionhaving the second conductivity type of dopants formed in the substrate104, specifically along the front surface 104A of the substrate 104. Thesecond conductivity type of the light-sensing region 106A is opposite tothe first conductivity type of the doped isolation feature 108 and theisolation well region 109. In the depicted embodiment, the light-sensingregion 106A is an n-type doped region. The pinned layer 106B is a dopedlayer overlapping the light-sensing region 106A at the front surface104A of the substrate 104. The pinned layer 106 has a conductivity typeof dopants opposite to the light-sensing region 106A. In the depictedembodiment, the pinned layer 106B is a p-type implanted layer. Thephotodetector 106 is surrounded by the doped isolation feature 108 andthe isolation well region 109.

The image sensor device 100 further includes a multilayer interconnect(MLI) 128 disposed over the front surface 104A of the substrate 104. TheMLI 128 includes various conductive features 132 and 130, which may bevertical interconnects, such as contacts and/or vias 130, and horizontalinterconnects, such as lines 132. The conductive features 130 and 132are formed by suitable process, including deposition, lithographypatterning, and etching processes to form vertical and horizontalinterconnects.

The various conductive features 130 and 132 of the MLI 128 are disposedin an interlayer dielectric (ILD) layer 134. The ILD layer 134 mayinclude silicon dioxide, silicon nitride, silicon oxynitride, TEOSoxide, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG),fluorinated silica glass (FSG), carbon doped silicon oxide, low-kdielectric material, or combinations thereof. The ILD layer 134 may beformed by suitable process, including spin-on coating, chemical vapordeposition (CVD) or plasma enhanced chemical vapor deposition (PECVD).In one example, the MLI 128 and the ILD layer 134 may be formed in anintegrated process including a damascene process.

In some embodiments, further process steps are included after the MLI128 formation. As illustrated in FIG. 2C, a carrier wafer 136 is bondedto the MLI 128. The carrier wafer 136 provides mechanical strength andsupport for processing the back surface 104B of the substrate 104. Aplanarization process, such as a chemical mechanical polishing (CMP)process, is applied to the back surface 104B of the substrate 104 toreduce a thickness of the substrate 104. A doped layer 138 is formed byan implantation process, diffusion process, annealing process orcombinations thereof through the back surface 104B. An antireflectivelayer 140, a color filter 142 and a lens 144 disposed over the backsurface 104B of the substrate 104. The color filter 142 and the lens 144are aligned with the light-sensing region 106A of the photodetector 106.

In the above depicted embodiments, image sensor device 100 includes ap-type doped substrate 104. Various doping configurations for variousfeatures, such as the light-sensing regions 106A, the doped isolationfeature 108, the isolation well region 109 and the floating diffusionregion 120, described above should be read consistent with the formationof the image sensor device 100 with a p-type doped substrate.Alternatively, image sensor device 100 may include an n-type dopedsubstrate 104 or an n-type material in the substrate 104. Various dopingconfigurations for various features described above should be readconsistent with the formation of the image sensor device 100 with ann-type doped substrate.

Various embodiments of the present disclosure may be used to improve theperformance of an image sensor device. For example, the doped isolationfeature 108 is formed in the pixel region 101 by implantation processes.This disclosure eliminates the drawbacks in conventional methods withetching damages during the shallow trench isolation (STI) formation inthe pixel region. Without etching damages, this disclosure could reducedark current or reduce white pixel defects of the image sensor device.In another example, the gate stack 114 is formed before the implantationprocess for forming the doped isolation feature 108. The gate stack 114provides a physical feature with a sharp edge for a better lithographyalignment in the mask layer 202 formation. The alignment of mask layer202 provides an accurate control of the overlay between the gate stack114 and later formed doped isolation feature 108. The implantation forthe doped isolation feature 108 is performed with a tilt angle θ. Thedoped isolation feature 108 may extends under the gate stack 114 alongthe first axis (along line B-B′) with a length L form the edge 114E ofthe gate stack 114. A possible current leakage path along the edge 114of the gate stack 114 from the drain region to the source region iseliminated. The short between source/drain regions 118B is prevented.

One aspect of the disclosure describes a method of forming an imagesensor device. An isolation well is formed in a pixel region of asubstrate. The isolation well has a first conductivity type. A gatestack is formed over the isolation well on the substrate. A mask layeris formed over the isolation well and covering at least a majorityportion of the gate stack. A plurality of dopants is implanted in thepixel region not covered by the mask layer to form doped isolationfeatures surrounding an active region. The gate stack is disposed on theactive region . The plurality of dopants has the first conductivitytype. A source region and a drain region are formed on opposite sides ofthe gate stack in the substrate. The source region and the drain regionhave a second conductivity type opposite to the first conductivity type.

A further aspect of the disclosure describes a method of forming animage sensor device. A pixel region has a front surface in a substrate.The front surface is defined by a first axis and a second axisperpendicular to the first axis. A gate stack is formed along the firstaxis on the front surface in the pixel region. The gate stack has alength X₁ along the first axis and a second length Y₁ along the secondaxis. A mask layer is formed over at least a majority portion of thegate stack and a portion of the front surface to define an active area.A plurality of dopants is implanted with a tilt angle θ into thesubstrate not covered by the mask layer to form doped isolation featuresin the pixel region. The doped isolation features surrounds the gatestack and the active area.

The present disclosure also describes an image sensor device. The imagesensor device includes a substrate having a first surface. An isolationwell region is disposed within the substrate. The isolation well regionis under the first surface with a distance W₁. A gate stack is disposedover the isolation well region on the first surface of the substrate.The gate stack has an edge. A doped isolation feature is disposed withinthe substrate and surrounding an active region. The gate stack isdisposed on the active region. The doped isolation feature has a topcorner aligned with the edge of the gate stack and extends from the edgeof the gate stack under the gate stack with a length L.

Although the embodiments and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed:
 1. A method of forming an image sensor device, themethod comprising: forming an isolation well in a pixel region of asubstrate, wherein the isolation well has a first conductivity type;forming a gate stack over the isolation well on the substrate; forming amask layer over the isolation well and covering at least a majorityportion of the gate stack; implanting, using the gate stack and the masklayer as masks, a portion of the pixel region to form doped isolationfeatures, wherein the doped isolation features have the firstconductivity type; and forming a source region and a drain region onopposite sides of the gate stack in the substrate, wherein the sourceregion and the drain region have a second conductivity type opposite tothe first conductivity type.
 2. The method of claim 1, wherein the stepof implanting is performed with a tilt angle θ to a front surface of thesubstrate from about 75 to about 90 degrees.
 3. The method of claim 2,wherein the doped isolation features extend from an edge of the gatestack under the gate stack with a length L.
 4. The method of claim 3,wherein the length L is less than about 0.1 μm.
 5. The method of claim1, wherein the mask layer covers the entire gate stack.
 6. The method ofclaim 1, wherein the isolation well is under a front surface of thesubstrate with a distance W₁, wherein the dope isolation features have adepth D₁ into the substrate, and the distance W₁ is substantially equalto the depth D₁.
 7. The method of claim 1, wherein the source region andthe drain region are surrounded by the doped isolation features.
 8. Themethod of claim 1, wherein the source region and the drain region areconfined in an area covered by the mask layer.
 9. The method of claim 1,further comprising: forming at least one photodetector in the pixelregion surrounded by the doped isolation features.
 10. A method offorming an image sensor device, the method comprising: providing a pixelregion having a front surface in a substrate, wherein the front surfaceis defined by a first axis and a second axis perpendicular to the firstaxis; forming a gate stack along the first axis on the front surface inthe pixel region, wherein the gate stack has a length X₁ along the firstaxis and a second length Y₁ along the second axis; forming a mask layerover at least a majority portion of the gate stack and a portion of thefront surface to define an active area; and implanting a plurality ofdopants with a tilt angle θ into the substrate not covered by the masklayer to form doped isolation features in the pixel region, wherein thedoped isolation features surround the active area.
 11. The method ofclaim 10, wherein the active area has a length X₂ along the first axis,and the length X₂ is not greater than the length X₁.
 12. The method ofclaim 10, wherein the tilt angle θ is in a range from about 75 degreesto about 90 degrees.
 13. The method of claim 10, wherein the dopedisolation features extend from an edge of the gate stack under the gatestack with a length L.
 14. The method of claim 13, wherein the length Lis less than about 0.1 μm.
 15. The method of claim 10, after the step ofimplanting a plurality of dopants further comprising: forming a sourceregion and a drain region on opposite sides of the gate stack in theactive area.
 16. The method of claim 10, further comprising: forming atleast one light-sensing region in the substrate surrounded by the dopedisolation features, wherein the at least one light-sensing region has aconductivity type opposite to a conductivity type of the doped isolationfeatures.
 17. An image sensor device, comprising: a substrate having afirst surface; an isolation well region disposed within the substrateand under the first surface with a distance W₁; a gate stack disposedover the isolation well region on the first surface of the substrate,the gate stack having an edge; and a doped isolation feature disposedwithin the substrate and surrounding an active area, wherein the gatestack is disposed on the active area, wherein the doped isolationfeature has a top corner aligned with the edge of the gate stack andextends from the edge of the gate stack under the gate stack with alength L.
 18. The image sensor device of claim 15, wherein the dopedisolation feature has a convex portion near the edge of the gate stack.19. The image sensor device of claim 15, wherein the length L is lessthan about 0.1 μm.
 20. The image sensor device of claim 15, theisolation well region and the doped isolation feature have a sameconductivity type.